Effects of xylene exposure on the metabolism of antipyrine in vitro and in vivo in the rat

Metabolic interaction and disposition of methyl ethyl ketone and m-xylene in rats at single and repeated inhalation exposures

1. Rats were exposed to m-xylene (300 ppm) and methyl ethyl ketone (MEK, 600 ppm) vapour, separately and in combination. 2. Repeated exposures to m-xylene enhanced liver drug-metabolizing capacity, whereas MEK showed no effects. After mixed exposure the cytochrome P-450-dependent monooxygenase activities were additively or synergistically induced. 3. In the presence of MEK the overall metabolism of xylene was strongly inhibited both after single and repeated exposures, an effect accompanied by elevation of xylene concentration in blood (18-29%) and fat (25-32%). 4. The 24-h excretion of the urine metabolites of m-xylene was decreased by 22-24% in mixed exposures: the excretion of methylhippuric acid was decreased (29%), but that of 2,4-dimethylphenol increased (9-35%). 5. After repeated inhalation exposures the excretion of xylene metabolites in urine was consistently higher, whereas the concentrations of xylene in fat (but not the concentration of MEK) were lower than after a single treatment, conceivably due to accelerated metabolic clearance of xylene. 6. Thioether excretion in urine was enhanced in xylene-treated rats (7-13-fold), but was not influenced by the induced changes in the metabolism of xylene. Xylene inhalation caused liver GSH to decrease slightly (10%), as did inhalation of MEK, but the latter did not enhance the excretion of thioethers. 7. MEK is a potent inhibitor of the side-chain oxidation of m-xylene producing methylhippuric acid, but not of its ring oxidation to 2,4-dimethylphenol, and exhibits a synergistic inducing effect on liver enzymes responsible for the oxidation of m-xylene. The increased ring oxidation of m-xylene was not associated with increased production of reactive metabolites indicated by GSH-depletion or thioether formation.

Influence of aromatic hydrocarbons on the metabolism of dichloromethane to carbon monoxide in rats

The influence of prior or simultaneous oral administration of benzene, toluene, o-, m-, or p-xylene on the carboxyhemoglobin (COHb) level after a single dose of dichloromethane (DCM) was investigated in male rats. Six hours after administration of DCM, 6.2 mmol/kg, the mean maximum COHb level was 9.3 +/- 1.9%. This level was significantly enhanced by prior administration of benzene (16.9 mmol/kg) at 12-24 h, of toluene (18.8 mmol/kg) at 20-28 h, of o- (16.6 mmol/kg) and m-xylene (16.3 mmol/kg) at 20-32 h, and of p-xylene (16.2 mmol/kg) at 24-32 h. The corresponding maximum COHb levels were 20.7 +/- 1.3, 18.6 +/- 1.1, 18.9 +/- 1.1, 22.7 +/- 1.2, and 13.2 +/- 1.0%, respectively. After simultaneous administration of both DCM and the aromatic solvent, the COHb formation was inhibited: values of 1.3 +/- 0.3, 1.7 +/- 0.4, 3.6 +/- 0.2, 1.9 +/- 0.2, and 2.0 +/- 0.2% COHb, respectively, were found. The inhibition was also evident when DCM was administered 12 h after toluene or m-xylene and 12, 16 or 20 h after p-xylene. The inhibition was dose-related as seen after combined gavage of o-, m-, or p-xylene and DCM. The o- and m-, but not the p-methylhippuric acid (MHA) excretion in the urine was significantly reduced after simultaneous administration of equimolar doses of DCM and the corresponding xylenes. In conclusion, it seems that the stimulation or inhibition of the COHb formation after DCM caused by pretreatment with or by simultaneous administration of the aromatic solvents is due to the induction of cytochrome P-450 IIE1 or to competition between DCM and the aromatic solvent on this isozyme of cytochrome P-450.

Metabolism of antipyrine and m-xylene in rats after prolonged pretreatment with xylene alone or xylene with ethanol, phenobarbital or 3-methylcholanthrene

1. The metabolic disposition of antipyrine (AP) and m-xylene (XYL) has been studied in rats pretreated for a prolonged period with XYL, dosed alone or in combination with ethanol, phenobarbital (PB), or 3-methylcholanthrene (MC). 2. XYL inhalation exposure at 300 ppm in air (7 h/day, 4 days/week, for 1 or 4 weeks) did not alter the total 24-h recovery of AP and its major metabolites in urine, but the excretion profile changed compared with controls: 3-hydroxymethylantipyrine (3-HMA) increased (less than or equal to 14%, P less than 0.001), norantipyrine (NORA) (less than or equal to 23%, P less than 0.01) and AP (less than or equal to 53%, P less than 0.01) decreased. 4-Hydroxyantipyrine (4-OHA) was unchanged. 3. Oral dosage of XYL at 800 mg/kg per day (5 days/week, for 12 days) altered the metabolic disposition of AP similarly to inhalation. 4. XYL + ethanol did not alter the xylene-type effect on AP metabolism. This was at variance with the changes following XYL + PB and, to a greater extent, XYL + MC pretreatments: 4-OHA increased (53-74%, P less than 0.01), 3-HMA (11-42%, P less than 0.05) and AP (greater than or equal to 50%, P less than 0.05) decreased. The effect on NORA was less clear. 5. XYL pretreatment accelerated metabolic disposition of its major urinary metabolite, methylhippuric acid (MHA) and formation of thioethers. 6. Thioether excretion in 24 h urine was enhanced about 10-fold after XYL inhalation and 20-fold after oral administration. Only XYL + PB treatment enhanced further the excretion of xylene-derived thioethers (P less than 0.05). 7. Drug-metabolizing activity (phase I and II reactions) in liver, lung and kidney showed that the treatments resulted in marked and differential biochemical alterations. 8. In conclusion, m-xylene enhanced the rate of its own metabolism and induced differential changes on urinary AP metabolite profile depending on the pretreatment.